SAS is a prominent technology for implementing enterprise storage systems and is primarily used as a hard disk drive (HDD) attachment technology within enterprise storage system designs. PCIe is a popular computer bus technology used in industry standard server designs and may be used as an interface for attaching high performance solid state disk (SSD) devices. The incumbent popularity of SAS as a scalable storage interconnect combined with recent proposals for high performance PCIe based storage protocols including small computer system interface (SCSI) express and non-volatile memory (NVM) express are driving a desire to intermix these storage technologies in new enterprise storage system designs.
SAS is a serial interconnect technology and PCIe is a input/output bus protocol. SAS and PCIe may have different performance characteristics and targeted applications.
SAS is a technology for transporting SCSI protocol, and it is a prominent technology for connecting storage devices such as HDDs to host bus adapters (HBAs) and redundant array of independent disks (RAID) controllers that use SCSI protocol as a higher layer data transfer protocol. SAS is popular because it is relatively low cost, is highly scalable, and provides high bandwidth. SAS protocol was originally conceived as an efficient and high-bandwidth interconnect for hundreds to thousands of relatively low performance, high-capacity HDDs. Numerous storage component vendors support SAS protocol in their components, and the majority of storage system vendors use SAS as the HDD interconnect in their system designs.
PCIe is also a high-bandwidth interconnect for general computer peripheral devices, and it provides very low latency data transfers between a host computer and a peripheral device. New storage protocols such as SCSI express and NVM express use PCIe as an underlying transport technology. These protocols are fundamentally intended for use with low latency storage devices such as SSDs. Low cost PCIe switches are readily available and can provide limited scalability to support fan out to multiple storage devices. However, PCIe lacks the scalability required for enterprise storage arrays and is not considered a suitable replacement for SAS as a transport technology for high capacity storage systems.
As PCIe is a packet-switched network it does not suffer from the same connection-blocking problem as SAS technology. However, high-capacity drives may not be developed for PCIe interfaces due to the prevalence of serial advanced technology attachment (SATA) interfaces on desktop, notebook, and server CPU chipsets.
In some implementations, it is desirable to have storage systems that support both high-performance PCIe storage devices and high-capacity SAS or SATA storage devices.
Market dynamics are driving some enterprise storage system designers toward designs that support intermixing storage devices with both SAS and PCIe interfaces in the same system.